In a fuel assembly for a nuclear power boiling water reactor (BWR), there are a number of fuel rods, which comprise a nuclear fuel material. The bundle of fuel rods are surrounded by a fuel channel, which forms a surrounding wall of the fuel assembly. When the fuel assembly is in operation in a nuclear reactor, a cooling medium, usually water, flows up through the fuel assembly. This water fulfils several functions. It functions as a cooling medium for cooling the fuel rods such that they will not be overheated. The water also serves as a neutron moderator, i.e. the water slows down the neutrons to a lower speed. Thereby, the reactivity of the reactor is increased.
Since the water flows upwards through the fuel assembly, in the upper part of the fuel assembly, the water has been heated to a larger extent. This has as a consequence that the portion of steam is larger in the upper part of the fuel assembly than in the lower part.
The internal pressure in the fuel channel during operation is higher in the lower part of the fuel channel than in the upper part of the fuel channel. It is known to produce a fuel channel which has a varying thickness. The thinner parts of the fuel channel are often obtained by some kind of working, where material is removed from the fuel channel wall, for example by milling.
U.S. 2006/0144484 A1 describes a method of producing a fuel channel. FIGS. 4 and 5 in this document show that first two U-shaped profiles are produced, which profiles then are welded together such that the fuel channel is formed.
U.S. Pat. No. 4,749,543 describes a fuel channel with a varying thickness.
U.S. Pat. No. 4,970,047 describes a fuel channel with a varying thickness, obtained by stepwisely shaving the inner surface of the channel box (see abstract).
DE 697 16 188 T2, JP 2-216087 and JP 53-43193 also show fuel channels with a varying thickness.